Optimal delivery of nutritional support during critical illness is central to appropriate intensive care unit management, and yet fundamental gaps in knowledge exist regarding timing, route, dose, and type of nutritional support for critically ill infants and children. Understanding how to optimize nutritional support during pediatric critical illness is important because even brief periods of malnutrition in infancy result in permanent negative effects on long-term neurocognitive development. Optimized nutrition support is a way to improve morbidity for survivors of pediatric critical illness. Parenteral nutrition (PN) supplementation could improve long-term neurocognitive outcome for pediatric critical illness by preventing acute malnutrition, but has unknown effects on intestinal barrier function; a proposed mechanism for late sepsis and infectious complications during critical illness.